The invention relates to a limited slip differential for an automotive vehicle and, more specifically, to a limited slip differential having a thermal compensating valve which regulates torque bias in the differential.
In applications that require a relatively uniform flow of pressurized fluid to a pressure-operated mechanism, such as a friction clutch, the actuating pressure typically is calibrated for operation within a predetermined temperature range. If the operating temperature of the fluid decreases to a low level outside the calibrated temperature range, the resulting fluid viscosity increase may adversely affect performance of the pressure-actuated mechanism. Conversely, if the operating temperature is higher than the calibrated temperature range, viscosity decreases in the hydraulic fluid may adversely affect performance.
If the pressure-actuated mechanism is a friction clutch in a limited slip differential mechanism for an automotive vehicle driveline, the clutch transfers torque between a differential side gear of the differential mechanism and a differential pinion carrier. To compensate for viscosity changes, it is necessary to account for temperature changes in the hydraulic fluid pressure delivered to a pressure chamber of the friction clutch. In the case of a limited slip differential mechanism for a rear wheel drive vehicle, the differential side gear thrust force may be relied upon to apply the clutch. The torque bias in the differential mechanism developed by the clutch during low temperature operation should emulate the torque bias that would normally exist at warmer temperatures for which the differential mechanism is calibrated.
Viscosity change compensation for the pressure fluid is needed to ensure reliable operation of the limited slip differential. A lack of viscosity change compensation could cause premature application of the clutch. In a limited slip differential for a rear wheel drive vehicle, this may contribute to unpredictable handling of the vehicle or cause so-called xe2x80x9ccrow hoppingxe2x80x9d of the vehicle during steering maneuvers. In the case of a limited slip differential for a front wheel drive vehicle, a lack of viscosity change compensation may tend to cause anxe2x80x9cunder-steerxe2x80x9d condition during steering maneuvers.
The side gears for a differential mechanism of this kind engage differential pinions that in turn are journalled on a pinion shaft or spider member that is supported by a differential carrier, the carrier in turn being driven by a crown gear. The side gears are connected respectively to each of two axle half-shafts for the vehicle traction wheels. Examples of differential mechanisms of this kind may be seen in U.S. Pat. Nos. 5,536,215, 5,595,214, 5,310,388, and 5,611,746, which are assigned to the assignee of this invention. Their disclosures are incorporated herein by reference.
The limited slip differentials disclosed in these reference patents include a speed sensitive torque bias wherein the bias torque is related to the difference in the speeds of the differential side gears and the pinion carrier. A lack of viscosity change compensation may adversely affect the speed sensitive bias as well as the torque sensitive bias.
The present invention is directed toward a limited slip differential for an automotive vehicle including a torque input differential pinion carrier, two differential side gears and a pair of torque output axle half shafts. The differential also includes at least one friction clutch assembly having first friction clutch plates connected driveably to the differential pinion carrier and second friction clutch plates connected driveably to one of the side gears. An annular pressure chamber is defined in the carrier. An annular piston is movably supported within the pressure chamber and adjacent the friction plates of the friction clutch assembly. The limited slip differential further includes a positive displacement gear pump having pumping elements defining high pressure and low pressure ports whereby pressure in the pressure chamber creates a force on the piston that engages a clutch assembly to effect a friction torque bias in the differential through the friction clutch assembly. Furthermore, the piston includes at least one flow control orifice through which fluid may flow from the high pressure port of the positive displacement gear pump via the annular pressure chamber and a thermal compensating valve that is mounted on the piston and is operable to decrease the flow area of the flow control orifice upon increasing temperature. Furthermore, the thermal compensating valve is also operable to increase the flow area of the flow control orifice upon decreasing temperature. In this way, the operation of the thermal compensating valve controls the valve of pumped hydraulic fluid from the pressure chamber and thereby controls the torque bias in the differential.
The thermal compensating valve in a limited slip differential environment may also include a second flow control valve in the friction clutch pressure chamber for independently controlling flow of pressurized actuating fluid for the clutch. In the preferred embodiment the thermal compensating valve may include a slide valve plate which slidably engages the base plate and is movable linearly relative to a base plate. Guide edges on the base plate are positioned on opposite sides of the slide valve plate. The slide valve plate has a flow metering edge positioned over the base orifice so that the effective fluid flow area of the orifice is varied as the actuator coil temperature changes. The valve on the piston provides a fluid flow path from the clutch pressure chamber to the low pressure fluid flow return circuit for the positive-displacement pump.